A turbine bucket is generally provided for a purpose of properly converting energy contained in thermal fluid into rotation energy. When designing the turbine bucket, it is necessary that the turbine bucket has a strength of withstanding a loading force and a centrifugal force by the thermal fluid and has to satisfy a mechanical characteristic with regard to vibration characteristic which prevents stimuli at the time of rated rotation. Further, in order to converting the thermal fluid energy into the rotation energy it is necessary to satisfy aerodynamic characteristic of reduced energy loss. Accordingly, in order to satisfy both the mechanical characteristic and the aerodynamic characteristic at the same time, it is necessary to overcome mutually contradicting structural requirements.
When there is a problem with regard to strength because of stress concentration at a contain position on a turbine bucket, even if a blade profile having a stream line reflecting fluid flow performance, it is necessary to thicken the blade cross section to increase the blade rigidity. Further, if the vibration characteristic of the blade profile shows stimuli at the time of the rated rotation which has to be avoided, it is also necessary to modify the blade profile. In particular, with regard to a turbine bucket for a steam turbine, if a higher efficiency of the blade performance is seeked, rigidity of individual blades is reduced, therefore, in order to increase rigidity of the blade structure as a whole, a blade connecting structure is employed in which adjacent blades are connected by such as shrouds and tie wires. Since such blade connecting structure disturbs the fluid flow in view of flow performance, the structure is not necessarily optimum as a turbine bucket as a whole.
In order to overcome these problems, it is necessary to determine the blade profile with only one solution for every limiting condition such as a blade length so as to fully satisfy reliability based on the mechanical characteristic as well as the aerodynamic characteristic. For example, U.S. Pat. No. 5,267,834 discloses a structure in which a blade profile satisfying strength, vibration and performance properly when the blade length is about 660 mm is determined, and a cover piece is provided at tips of the blades and a sleeve is provided at intermediate portions of the blades and the adjacent blades are connected by a member connecting the adjacent blades at two positions in the radial direction.
In the above referred to U.S. Pat. No. 5,267,834, it is indicated that for the blade profile and the blade structure when the blade length is about 660 mm through the provision of the blade connecting member at two positions in the radial direction the rigidity of the blade structure as a whole is enhanced. However, the provision of such blade connecting member at two positions in the intermediate portions of the blades disturbs working fluid flow at substantially the intermediate portions between the blades and extremely reduces fluid flow performance representing aerodynamic characteristic at the intermediate portions.
The present invention is carried out in view of the above problems and an object of the present invention is to provide a turbine bucket in which adjacent blades are connected without using the connecting member at the intermediate portions of the blades.